1. Field of the Art
This invention relates to an optical beam scanner suitable for use in laser beam printers, laser-engraving of printing blocks or the like, and more particularly to an optical beam scanner capable of making corrections for irregularities in pitch of beam scanning, which are caused in a direction perpendicular to the transverse scanning direction due to inclination or angular deviations of reflective deflection surfaces like reflecting facets of a polygon mirror.
2. Description of the Prior Art
There has been known in the art an optical laser beam scanner which employs a first image-forming optical system, reflective deflection surfaces on a rotating polygon mirror or the like and a second image-forming optical system (f.theta. lens) successively in that order, thereby forming an image of a laser light beam from a light source linearly on or in the vicinity of a reflective deflection surface by the first optical system, scanning the laser beam mechanically by rotation of the polygon mirror, and converging the laser beam through the second optical system to scan a surface in the fashion of the so-called beam spot scanning. A problem with this sort of laser beam scanner is that errors in parallelism of the reflective deflection facets of the polygon mirror with the axis of its rotational center shaft, namely, inclination or angular deviation of the reflective deflection surfaces give rise to irregularities in pitch of beam spot scanning in a direction perpendicular to the transverse main scan direction on the scanning surface or in a subscan direction, giving adverse effects on the picture image of the scanning line picture image.
As countermeasures for such inclination of the reflecting facets or surfaces, there have been proposed various optical means which are designed to correct and improve the facet inclination by optically relocating the reflective defection surface and the scanning surface into conjugative positions.
For example, Japanese Laid-Open Patent Application 48-98844 proposes an optical system which employs, in the second image-forming optical system between a reflective deflection surface and a scanning surface, a spherical lens system having distortion characteristics for realization of uniform speed scanning and an elongated cylindrical lens. Also an optical system which employs a cylindrical mirror in place of the above-mentioned cylindrical lens is disclosed in Japanese Laid-Open Patent Application 61-84620.
However, in case of an optical system which is composed of a spherical lens and an elongated cylindrical lens as in Japanese Laid-Open Patent Application 48-98844, it is necessary to locate the cylindrical lens closely to the scanning surface for correction of field curvature which would otherwise occur in a considerable degree, and to provide a longer cylindrical lens to cover a broader scan width, resulting in a difficulty of arranging the optical system in a compact form. In case the cylindrical lens is substituted with a cylindrical mirror, the degree of field curvature can be improved slightly as compared with that of the cylindrical lens, and the cylindrical mirror can be located at a greater distance from an inspecting surface for the sake of compact construction. However, the cylindrical mirror which serves to turn a light beam is naturally subject to certain positional restrictions inherent to the mirror action.
On the other hand, as shown in U.S. Pat. No. 4,379,612 and Japanese Laid-Open Patent Applications 57-35823 and 63-50814, attempts have been made to realize a compact optical system by incorporating a lens with a corrective toric surface into a lens system with distortional characteristics for uniform speed scanning, dispensing with the elongated cylindrical lens or mirror. In this case, the second image-forming optical system is composed of a couple of lens elements, namely, successively from the side of the reflective deflection surface, a first singular lens element provided with a spherical or cylindrical surface and a second singular lens element provided with a toric surface and having positive refractive power in the plane of scanning. However, with an optical system of this type, it is difficult to shorten further the light path from the scanning optical system to the scanning surface for the purpose of making the apparatus compact because the back focal length of the lens system on the side of the scanning surface becomes longer than the focal length.